The present invention relates to a video signal generator and, more particularly, to a circuit for successively generating a series of radial sweep lines which simulate a rotating sweep line of the type characteristic of many video radar displays.
The Weather Service has a number of weather radar stations throughout the United States, each presenting a radar picture which is transformed from rho-theta to direct linear raster scan display by a vidicon camera. Essentially, this is done by focusing the camera on the display tube of a radar unit. A transparent overlay is provided on the weather radar tube which provides an outline of a map of topological features, such as shore lines, and state boundaries, with range circles which correspond to the range at which the radar device is operated. This overlay also provides a surface for notes made by the meteorologists operating the radar unit.
The radar picture is transmitted over 3 KHz bandwith telephone lines at a rate of one frame every one hundred seconds, and is intended to be displayed on a storage tube at the receiving location, or to be reproduced by a facsimile machine operating at eight lines per second. The single frame of low frequency picture information is thus comprised of eight hundred lines of video information, together with horizontal sync components which indicate the end of each line, and a vertical sync component which identifies the end of the frame.
There is substantial demand for current weather information by the public through television broadcasting stations, or through television cable systems. The use of a facsimile type picture signal, however, is an impediment to full utilization of this weather radar information, since this type of facsimile receiving equipment is often not as convenient to use. It is much more convenient to display the weather radar picture with an ordinary television monitor.
In order to convert the available facsimile picture to a standard video display signal, a converter, disclosed in U.S. Patent Application Ser. No. 897,281, filed Apr. 17, 1978, by James Dalke, and assigned to the assignee of the present invention, has been developed. In this converter, one frame at a time is converted into digital signals. The digitally converted video signals are stored in a random access memory in a 256.times.256 matrix format, ultimately providing the picture at a 1:1 aspect ratio. The memory is then read out at a higher speed, synchronized to provide broadcast compatible television signals. The output is converted to an analog signal, appropriate level adjustment is made, and color is added to the video display.
It has been felt that the stationary picture displayed on the television receiver lacks dynamic interest. In order to introduce action into the picture, the above noted application disclosed a sweep generator which provides a rotating sweep line video signal which is added to the displayed video radar picture. This sweep line is superimposed on the radar display purely for ascetic purposes and does not relate to the actual sweep of the weather radar unit producing the picture.
The approach taken in the above referenced Dalke application to generate the rotating sweep line video is to provide a rotating disc having a sweep line marked thereon. A conventional vidicon camera monitors the rotating disc to produce a rotating sweep line video output. This video output is then added to the static picture produced by the video converter. Such a sweep line generator is both bulky in size and expensive.
Accordingly, it is seen that there is a need for a less expensive solid state sweep line function generator, capable of producing a video representation of a sweep line.